1. Field of the Invention
The use of X-rays in many fields for many purposes is almost common-place in the present day and age. One such use is in the field of medicine, wherein X-ray diagnosis and treatment is extremely important. Particularly where treatment is concerned, the use of X-ray generating equipment for extended periods of time is becoming more necessary.
X-ray generating equipment is fairly standard in construction. The main component is a generating tube. The major elements of this tube are a cathode, an anode with a target and a vacuum sealed glass, quartz or glasslike, enclosure. The cathode of the tube is similar to the filament in an ordinary light bulb. When energized, the cathode becomes heated and thus is caused to emit electrons. The electrons are attracted to the target which is part of the anode.
The anode includes the target which is generally a metal disc having a diameter of 3 to 5 inches and a thickness of 1/4 of an inch to 1 inch. The target has an outer most radius which is angled slightly outward and thus the target as a whole has the appearance of a very shallow truncated cone. The center of the diameter of the target includes a aperture through which a stem is positioned and fastened. The stem is mounted in a bearing structure so that the target and stem are free to rotate. The stem is further provided with appropriate apparatus in order to operate as a rotor in an electric motor. The stator, which completes the electric motor, is external to the X-ray tube and is positioned directly around the stem and its attachments.
An electric field is present between the cathode and anode when the X-ray tube is operating. It is this electric field which causes the electrons from the cathode to be attracted to the anode as noted above. When the electrons strike the target of the anode, X-rays are emitted therefrom. As further noted above, the extreme outside radius of the target, from 1/2 inch to 1 inch, is angled outwardly. This is precisely the area in which the electrons from the cathode strike the target. Thus the emitted X-rays are caused to be directed away from the angled portion of the target and through the glass enclosure. At this point, the X-rays may be utilized in any fashion desired.
While a considerable quantity of X-rays are caused to be emitted from the target as a result of the electrons striking it, a larger quantity of electrons do not result in X-ray emission but rather are stopped by the target causing a heating thereof. The temperature at the spot, and the surrounding area, on the target at which electrons strike quickly rises. For exactly this reason the target is arranged to rotate as noted above at a speed of from 3,000 rpm to 9,000 rpm. The rotation has two purposes.
First, as the target rotates, a different spot is subject to the striking electrons at each instant in time. In this manner, the temperature of the entire target is raised equally. Since the target does rotate, no feasible way has been found to carry the build up of heat away from the target.
Second, while the target rotates, the particular areas which are not under bombardment by electrons from the cathode are given the opportunity to cool through radiant dissipation of the heat.
Even though some heat is dissipated through radiant energy, the heat build up in the target is continually greater than the amounts dissipated. In this respect, the temperature of the target increases to the range of 1000.degree. to 1500.degree.C at which time the tube must be shut down or permanent damage may be done. If the tube is allowed to overheat, the target, or more particularly the bearings of the stem, may be rendered inoperative permanently.
2. Prior Art
For many applications of X-ray generation, and in particular the medical field, extending the time for which the tube may be used would be extremely advantageous. Therefore, manufacturers of X-ray equipment have endeavored to design systems which can operate for prolonged periods.
In this regard, the target in an X-ray tube was, at one time, constructed totally of a material which would emit X-radiation upon bombardment by electrons, i.e., tungsten or tungsten alloy. However, through experimentation it was discovered that such a target did not, in fact, display optimum thermal characteristics.
In an effort to increase heat absorption, a "layered" target was devised. This type of target included only a thin layer of X-ray generating material, i.e., tungsten alloy, while having the remainder of the target constructed of a material with a much higher specific heat, e.g., molybdenum or molybdenum alloy.
Due to the fact that the improved target with the X-ray generating layer is composed of layers of different materials and the different materials may be differently effected by heat, warping of the target was often the next problem encountered. To this effect, a solution to the problem of warping targets was sought after. U.S. Pat. No. 3,790,838 discloses one such solution in the way of a "warpless" target.
Another method utilized to effect increased efficiency of X-ray targets and thus tubes is the use of nonmetallic materials in the targets. The use of such nonmetallic materials, e.g., carbon, may appear in the form of a backing for the target or an integral portion thereof. An example of the latter is disclosed by U.S. Pat. No. 3,753,021. This particular patent has an outer conventional target ring of molybdenum or other suitable material and an inner heat sink ring, or ring segments, arranged to be in very close, tight proximity with the target ring. The heat sink ring is constructed of a material capable of storing large quantities of heat for extended periods and may be a material such as graphite or beryllia.
As mentioned above, additional problems arise when differing materials are utilized in a target due to the different reactions from extreme heat exposure. In this respect, the advantage of the last mentioned patent is derived from the ability of the heat build-up in the target material to be transfered to the heat sink material through a close, tight proximity contact. However, this close, tight proximity contact may in fact be less than intended due to different expansion rates of the materials. To some extent, the effects of this problem may be cancelled by other construction techniques such as segmenting the heat sink. Again, while the use of nonmetallic materials in a target increases the usefulness to some extent, the emphasis is to ever increase the effectiveness of the equipment.